In a large Dutch oven or casserole, combine the veal, chicken stock, wine, onions, carrots, celery, rosemary, bay leaves, and peppercorns. Add water as needed to cover the meat and bring to a boil. Reduce the heat to a simmer and cover tightly. Simmer, turning once, until the meat is tender and an instant-read thermometer registers 145 to 150 degrees F, about 50 minutes to 1 hour.
Transfer the veal to a platter and let cool. Strain and reserve the cooking broth. Wrap the veal tightly and refrigerate until well chilled (up to 24 hours).
To make the sauce, combine the tuna, anchovies, capers, white wine vinegar, lemon juice, and lemon zest in a blender or food processor and blend to a very smooth texture, about 2 minutes. Add the mayonnaise and process to blend thoroughly. Add 1 tablespoon of the reserved cooking broth. Add more, if desired, to taste or for a thinner consistency. Refrigerate in a tightly sealed container for at least 2 and up to 24 hours. (The sauce tastes best if made 1 day in advance and allowed to rest overnight).
To serve, thinly slice the meat and arrange on a platter. Drizzle the sauce over each slice, season generously with the kosher salt and black pepper. Garnish by sprinkling with additional capers and chopped parsley. Arrange the lemon slices around the platter and serve.
1 large egg, at room temperature*
1 tablespoon fresh lemon juice
1 cup olive oil
1/2 teaspoon salt
1/8 teaspoon freshly ground black pepper
In a blender or food processor, combine the egg and lemon juice and process for 10 seconds. With the machine running, slowly pour in the oil through the feed tube and process until emulsified. Add the salt and black pepper and pulse to blend. Transfer to an airtight container and refrigerate for at least 30 minutes before using. (The mayonnaise can be stored in an airtight container in the refrigerator for up 24 hours).
Yield: 1 1/4 cups